What happens when you write to a nil map vs an empty map?

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Accepted Answer

// Nil map — declared but not initialized
var m map[string]int

// Reading is SAFE — returns zero value
val := m["key"] // val = 0, no panic
fmt.Println(val)

// Checking existence is SAFE
v, ok := m["key"] // v=0, ok=false
fmt.Println(v, ok)

// Writing PANICS
// m["key"] = 1 // panic: assignment to entry in nil map

// Empty map — initialized, safe for writes
em := make(map[string]int)
em["key"] = 1 // safe

// Map literal — also safe
em2 := map[string]int{"a": 1, "b": 2}
em2["c"] = 3 // safe

// Common bug in structs
type Cache struct {
    data map[string]string
}

func NewCache() *Cache {
    return &Cache{data: make(map[string]string)} // always initialize
} Always initialize maps in struct constructors — do not rely on callers to initialize Use make(map[K]V, hint) with a size hint when the approximate number of entries is known Deleting from a nil map does NOT panic — delete() on nil map is a no-op len() on a nil map returns 0 without panic Range over a nil map produces zero iterations without panic

What happens when you write to a nil map vs an empty map?

Writing to a nil map causes a runtime panic. Reading from a nil map returns the zero value safely. An empty map initialized with make() is safe for both reads and writes.

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